Horses don't like milkweed any more than they like those daisies in my previous post. But monarch butterflies do. In fact, monarchs need milkweed. (Brace yourselves; science Googling ahead.) They lay their eggs on the plant, the caterpillar larvae feed on the plant, they form their coccoons there, and the butterfly sips its nectar, along with a few other flowers. In fact, the butterflies even gain a unique protection from eating milkweed as larvae - they take in cardiac glycosides, which makes them taste bad and is poisonous to birds. (I don't know how, but I'm guessing the cardiac part is a clue.)
Monarchs migrate, the only butterfly that does. Last year storms in Mexico killed many of the butterflies. It takes 2 or 3 generations for the surviving monarchs to get this far north, but I haven't seen a single monarch yet, and this is when they should be here. (Have you? Let me know!) For those who make it, I've got a smorgasbord of milkweed waiting in my pasture.